In hydraulic fracturing of wells, large pumps are connected to a wellhead by temporary flow lines to pump high volumes of fluid into the well. The pressure is selected to be high enough to cause cracks or fracturing of the earth formation. Valves are employed in the flow lines to control the flow.
One type of valve has a body with a flow passage extending through it that intersects a central cavity. A rotatable valve element, such as a cylindrical plug, is mounted in the cavity. The valve element has a passage that aligns with the passage in the body when open. A drive mechanism is used to rotate the valve element between open and closed positions.
The drive mechanism may have a drive sleeve that is rotated by a hand wheel attached to a worm gear. A drive shaft fits within the bore of the sleeve and has an inner end coupled to the valve element. A shear key inserts between mating grooves between the drive shaft and the drive sleeve. Rotating the hand wheel thus causes the drive shaft to rotate the valve element. The drive mechanism may have a face with indicia to indicate the open or closed position of the valve element. An indicator is mounted to the drive sleeve to align with the indicia.
High fluid pressure within the valve can require considerable force to rotate the hand wheel. If the force is too high, the shear key will shear, allowing the drive sleeve to rotate even though the valve element is not rotating. The indicator may indicate an erroneous position of the valve element because the indicator continues to rotate with the drive sleeve after the shear key shears.